1. FIELD OF THE INVENTION
The present invention relates to a process for the preparation of dialkyl carbonates by the reaction of carbon monoxide (CO) with alkyl nitrites in the presence of a catalyst from the group of the platinum metal halogenides on a support of alumosilicate zeolites with acidic centres, which may contain additional compounds of further elements.
Dialkyl carbonates are of general chemical significance. For example, diethyl carbonate is an excellent solvent in the medium boiling point range. Furthermore, the dialkyl carbonates are excellent carbonylation and acylation reagents. Finally, they are very important in the preparation of other carbonates, of urethanes and of ureas.
2. DESCRIPTION OF THE RELATED ART
Known preparations of dialkyl carbonates involve the reactions of phosgene or of alkyl chloroformates with alcohols. However, there is an increasing interest in replacing the use of the toxic phosgene or of the intermediate products derived therefrom, such as the chloroformic esters, by other processes. In addition to attempts to obtain dialkyl carbonates by reacting CO with lower alcohols, processes of particular importance are those in which CO is reacted in the gas phase with alkyl nitrite on a platinum metal catalyst. In reactions of this type, in addition to the desired dialkyl carbonate, dialkyl oxalate is always detected.
Thus, EP 425 197 discloses a process which, according to its preferred embodiment, leads to dialkyl carbonates of methanol or ethanol from CO and methyl or ethyl nitrite in the gas phase on a PdCl.sub.2 catalyst on activated carbon. The selectivities in obtaining the desired lower dialkyl carbonates, according to said EP 425 197, Table 1, reach levels of up to 94%; however, lower dialkyl oxalates and CO.sub.2 are always observed as by-products. Moreover, when this work was repeated, the high selectivities quoted could not be reproduced satisfactorily. The catalysts of said EP 425 197 contain additions of chlorides of base metals; a considerable addition of hydrogen chloride, namely an amount of 1 to 50 mol%, based on the platinum metal in the catalyst, is added to the system, or part of the catalyst has to be withdrawn from the reactor and subjected to a treatment with hydrogen chloride.
In the Journal for Catalytic Research (China), Vol. 10 (1), (1989) pp. 75-78, too, the support used for a PdCl.sub.2 -containing catalyst comprises charcoal, producing dimethyl carbonate from CO and methyl nitrite, but in each case dimethyl oxalate is also produced.
A Pd/charcoal catalyst is also mentioned in Chin. Sci. Bull. 34 (1989), 875-76 for the preparation of dimethyl carbonate from CO and methyl nitrite.
This preference for a charcoal support is not surprising, since it is reported in Platinum Metals Review 34 (1990), 178-180, referring to older literature, that in the reaction of a lower alkyl nitrite with CO on a Pd catalyst, depending on the support, different main products are obtained; according to this article, a charcoal support predominantly yields the dialkyl carbonates, while an oxidic support such as, for example, an Al.sub.2 O.sub.3 support primarily yields dialkyl oxalates.